1. Field of the Invention
The present application relates to hearing aids. More specifically, it relates to battery powered hearing aids comprising remote control receivers. The invention further relates to a method of operating a hearing aid. The invention also relates to a hearing aid system comprising a hearing aid and a remote control.
Present-day hearing aids are powered by tiny battery cells, preferably of the zinc-air variety. Zinc-air battery cells comprise a zinc anode, an aqueous alkaline electrolyte and an air cathode. Power is derived from the chemical reduction of oxygen, derived from the surrounding air, at the cathode, and the oxidation of zinc at the anode. Such a cell has the advantages of a very high power density, a comparatively constant power profile, and an environmentally friendly chemistry. The alkaline electrolyte in the cell is protected from the surrounding atmosphere by an airtight seal until employment, when the seal is broken prior to placing the cell in the battery compartment of the hearing aid, and the cell starts providing electrical power to the hearing aid.
The battery seal is usually embodied as a small label attached to the cathode of the battery cell, and the cathode terminal has tiny holes placed below the label to allow air to enter the interior of the battery cell when the label is removed. When air comes into contact with the electrolyte inside the battery cell, the electrochemical reaction is initiated, and an electric voltage difference is built up and maintained between the battery cell electrodes for the duration of the electrochemical reaction inside the cell. The typical voltage of a zinc-air battery cell is from 1.1 V to 1.4 V.
If left disconnected from any circuitry, and thus being without any external load after the seal is broken, the zinc-air battery cell is slowly depleted by a process known as self-discharging, and the cell will eventually lose its power over the course of a couple of days. This self-discharging is mainly the result of the electrolyte in the cell drying up, but other factors, like high humidity, or the presence of oxygen or carbon-dioxide in the vicinity of the cell, also affect the rate of self-discharge.
2. Prior Art
A common procedure for turning off a battery-powered hearing aid when not in use is by disconnecting the battery cell from the hearing aid circuitry, either by means of a power switch or by dislocating the cell itself from at least one of the battery terminals of the hearing aid, thus opening the electric circuit. Hearing aids may also employ a double-pivoted, swiveling battery compartment assembly in order to provide both a battery dislocation function for turning the hearing aid on or off, and an opening function for replacing the battery cell.
Both mechanical switches and battery terminals in hearing aids are prone to wear when the hearing aid is turned on and off many times. Battery terminals and contact elements of switches are preferably made from spring steel or phosphor bronze bent into the desired shape and subsequently gold-plated in order to prevent corrosion, but the physical dimensions of the hearing aid severely limit the obtainable durability of mechanical switches and battery terminals within the hearing aid, and the double duty performed by the battery compartment assembly, i.e. when changing the battery cell and when powering the hearing aid on and off puts a considerable amount of stress upon the battery terminals.
Electronic power switches are used in many types of electronic devices, usually in the form of a semiconductor element controlling the power circuit of the electronic device relying on a trigger impulse from a switch or the like. This type of circuit has a prolonged service life when compared to similarly employed mechanical switches, but it draws a modest amount of leakage current while the device is switched off. In a hearing aid, where the available power is limited, any significant leakage current would obviously shorten the service life of the cell, and an electronic power switch of this kind is thus a good choice for employment in a hearing aid.
Due to the dimensional restrictions mentioned in the foregoing, any switches in the hearing aid have to be made very small in order to fit into the hearing aid casing. Apart from being prone to wear and breakage, tiny mechanical switches may also be difficult to operate properly, e.g. by physically disabled hearing aid users. Power switches operated by dislocating the battery cell from the battery terminals of the hearing aid may also result in the cell falling out of the battery compartment by accident and eventually getting lost as the result of an erroneous operation by the user.
Remote control devices for use with hearing aids are known. They offer a convenient way of operating various user-accessible features of a hearing aid such as volume level and program selection, but they still require the hearing aid to be switched on in order to receive and process the commands transmitted from the remote control device.
An active command for controlling the power in a hearing aid from a remote control device is not easily employed. Obviously, if all circuitry in the hearing aid is powered off, no means for powering the hearing aid back on again by a corresponding command from the remote control device would have any effect. However, if only parts of the circuitry were powered off by such a command, i.e. all but the parts responsible for receiving and interpreting commands from the remote control device, the hearing aid could be cycled between a normal mode of operation, drawing full power from the hearing aid battery cell, and a stand-by mode, drawing very little power from the battery cell.
By definition, a stand-by mode of an electronic device is a mode of operation in which the electronic device consumes very little power when compared to the power consumption during normal operation, and from which mode the device may be brought into normal operation by performing some special action, e.g. activating an ‘on’-function, either directly by interacting with the circuitry of the electronic device, for instance by pushing a button or activating a switch, thus closing part of the circuit, or indirectly by transmitting a predetermined signal from a distance to a receiver located within the electronic device and being capable of interacting with the circuitry of the electronic device upon reception and detection of the predetermined signal, said receiver remaining active during the stand-by mode.
DE-B3-102006024713 proposes a hearing aid having means for detecting the presence of a passive, resonant circuit, comprising a capacitor and an inductor, in close proximity to the hearing aid. In one embodiment, the hearing aid has means for switching off its power when located close to the resonant circuit, e.g. when the hearing aid is placed in its storage box, the storage box having said resonant circuit embedded into its bottom wall. The means in the hearing aid for detecting the presence of a passive circuit comprises a transmitter and a transceiver coil. A part of the hearing aid is thus being disabled whenever the hearing aid is placed in its storage box. When the hearing aid is removed from the storage box, the power is reapplied to the disabled parts of the hearing aid.
The transmitter in the hearing aid according to the prior art emits short bursts of electromagnetic energy at the resonant frequency of the passive circuit through the transceiver coil, even when the hearing aid is supposed to be powered off. The electromagnetic energy excites the passive circuit to oscillate at the resonant frequency. While the passive circuit oscillates, it dissipates the absorbed energy as electromagnetic waves. These electromagnetic waves may then be picked up by the transceiver coil and detected by the hearing aid circuitry.
The range of a secure detection of the presence of a passive resonant circuit is limited by the amount of energy the transmitter is capable of dissipating. This puts a serious constraint to the detection range of the system, as the energy transmitted follows the inverse square law, i.e. the electromagnetic energy dissipated by the transceiver coil and the energy reflected back to the transceiver coil by the passive circuit decreases with the distance squared.
Taking the limited energy available in the hearing aid battery into account, the effective range for detecting a passive circuit by a transmitter in a hearing aid is, at the very best, only a few centimeters. When the transmitter in the hearing aid has to operate continuously in order to detect the presence of the passive resonant circuit, a considerable amount of current is consumed by the hearing aid even when it is supposed to be powered off.
Wireless receivers for controlling a stand-by mode may comprise receiver types capable of detecting acoustical, optical or electromagnetic signals generated by a suitable, corresponding transmitter. Acoustical transmission usually involves ultrasonic transducers unsuitable for use in a hearing aid due to limitations in size and power requirements. Optical transmission usually involves low-power infrared light emitting diodes, but such designs are dependent of a clear line-of-sight between the transmitter and the receiver, difficult to obtain in hearing aids being worn behind or in a user's ear.
Electromagnetic transmission, on the other hand, is well suited for use in low-power applications, as the power requirements of an electromagnetic receiver can be designed to be very low indeed. A transmitter may also be carried out of the line-of-sight as long as the receiver is within the detection limit. Electromagnetic remote control signals may further be modulated in a variety of ways suitable for the intended use, but a discussion of modulation techniques is beyond the scope of this application.
A hearing aid capable of entering a stand-by mode initiated by a remote control from a range of one to one-and-a-half meter would be desirable. Furthermore, a stand-by mode having the hearing aid drawing very little power from the battery cell within the hearing aid would be even more desirable.